1.4: Entry-page branch: / → /onboard or /work

[nextlevelshit]

Part of Epic #1565. Phase 1, depends on 1.2 (#1576+), 2.3 (later).

Webui root redirects to /onboard when sentinel .agents/.onboarding-done is missing, else /work. Demotes /runs from default landing.

Files:

• Modify: internal/webui/routes.go

Acceptance:

• Sentinel-aware root redirect (uses PRE-2 IsOnboardedAt)
• /runs reachable but not default
• Real browser smoke

Pipeline: impl-issue (--adapter claude --model cheapest)